The present invention relates to a dispersion compensating optical fiber used in, for example, wavelength division multiplexing transmissions, and a wavelength division multiplexing transmission line using a dispersion compensating optical fiber.
Single mode optical fibers having zero dispersion at a wavelength band of 1.3 xcexcm have been laid in the world over as transmission networks for optical transmissions. Recently, the amount of transmission information has been remarkably increased in line with development of an information society. Further, in line with an increase in such information, a wavelength division multiplexing transmission (WDM transmission) has been widely accepted in the field of transmissions, wherein now an age of wavelength division multiplexing transmission has arrived. The wavelength division multiplexing transmission is a system in which the wavelength of optical transmission is not a single mode but wherein optical signals are transmitted with the wavelength divided into a plurality, and the system is an optical transmission system suitable for high bit rate and high rate transmissions.
However, in a case where a wavelength division multiplexing transmission is carried out using a wavelength band of 1.3 xcexcm while using the existing single mode optical fiber having zero dispersion at a wavelength band of 1.3 xcexcm, since the wavelength band (for example, 1.52 through 1.62 xcexcm) of 1.55 xcexcm which is a gain band of normal optical amplifiers is not coincident with the wavelength band of 1.3 xcexcm, normal optical amplifiers cannot be used. Further, since the transmission at the wavelength band of 1.3 xcexcm is large, there remains a problem which hinders long-haul optical transmissions. In order to avoid this problem, researches have been carried out, in which wavelength division multiplexing transmissions are executed at a wavelength band of 1.55 xcexcm using the existing single mode optical fiber having zero dispersion at a wavelength band of 1.3 xcexcm.
However, if an optical transmission is performed at a wavelength band of 1.55 xcexcm using a single mode optical fiber having zero dispersion at the wavelength band of 1.3 xcexcm, the dispersion of signals of respective wavelength division multiplexing wavelengths is increased as optical signals propagate in a transmission single mode optical fiber since the existing single mode optical fibers positive dispersion and positive dispersion slope at the wavelength band of 1.55 xcexcm is increased. Therefore, it becomes difficult to separate signals at the receiving side, whereby the quality of optical transmission is worsened, and such a problem occurs, by which the reliability of optical transmission is lost.
Accordingly, recently, in order to solve these problems, a dispersion compensating optical fiber has been developed. The dispersion compensating optical fiber has negative dispersion. If the dispersion compensating optical fiber is connected to the receiving side of a single mode optical fiber, positive dispersion of optical signals transmitted through the single mode optical fiber is nullified by negative dispersion of the dispersion compensating optical fiber, wherein the optical signals are received in a state where the dispersion thereof at the receiving side becomes an almost zero dispersion. Thus, by connecting a dispersion compensating optical fiber to a single mode optical fiber, the respective wavelength division multiplexed optical signals can be separated at the receiving side, whereby a high bit rate and high rate optical transmission of high quality can be expected.
When using this type of dispersion compensating optical fiber, dispersion is compensated by connecting as short a dispersion compensating optical fiber as possible to the receiving side of a single mode optical fiber. To suit this usage, a dispersion compensating optical fiber developed previously is generally produced in the form of a module. As one of the examples, there is proposed a concept of an optical fiber suitable for a dispersion compensating optical fiber module disclosed in, for example, Japanese Laid-Open Patent Publication No. 11620 of 1994.
However, Japanese Laid-Open Patent Publication No. 11620 of 1994 does not provide any information regarding a profile of detailed refractive indexes of a dispersion compensating optical fiber. For this reason, it was impossible to specify the optimal refractive index profile of such dispersion compensating optical fibers as described above.
Further, in order to compensate dispersion by a short fiber length with a dispersion compensating optical fiber produced in the form of a module, it is necessary for a moduled dispersion compensating optical fiber to have high negative dispersion and high dispersion slope. If it is attempted that a dispersion compensating optical fiber has high negative dispersion and high dispersion slope, conditions of various types of parameters to define the refractive index profile of a dispersion compensating optical fiber are made severe, whereby it becomes very difficult to produce such dispersion compensating optical fibers. Moreover, if a dispersion compensating optical fiber is of such a refractive index profile structure as to have high negative dispersion and a high negative dispersion slope, a non-linear phenomenon is necessarily likely to occur, and the mode field diameter (MFD) of optical fibers is decreased. If the non-linear phenomenon occurs, the signal waveform may be distorted, which will cause another problem in achieving a high bit rate and a high rate in wavelength division multiplexing transmissions.
Further, as the mode field diameter of the optical fiber is decreased, still another problem arises, in which the non-linear phenomenon is likely to occur or the connection loss is increased when the dispersion compensating optical fiber is connected to a single mode optical fiber.
On the other hand, another optical transmission system is proposed, in which optical transmissions are carried out by using a dispersion shifted optical fiber in which the zero dispersion wavelength is shifted from a wavelength band of 1.3 xcexcm to a wavelength band of 1.55 xcexcm. By transmitting optical signals at a wavelength of 1.55 xcexcm using a dispersion shifted optical fiber having zero dispersion at a wavelength band of 1.55 xcexcm, it becomes possible to transmit signals free from any dispersion. However, in general, since a dispersion shifted optical fiber has a dispersion slope, no dispersion is produced for a zero dispersion wavelength at the wavelength band of 1.55 xcexcm if a wavelength division multiplex transmission is carried out using signals at a wavelength band of 1.55 xcexcm, but dispersion is produced with respect to signals in the vicinity of the wavelength band of 1.55 xcexcm. Therefore, the optical transmission system in which a dispersion shifted optical fiber is used is not suitable for a high bit rate and high rate wavelength division multiplexed optical transmissions. Also, this type of dispersion shifted optical fiber has such a tendency, whereby a non-linear phenomenon is likely to occur, differing from a single mode optical fiber.
Therefore, the present inventor changed the conventional idea from a dispersion compensating optical fiber being made into a moduled optical fiber exclusive for dispersion compensation to an optical fiber which has both functions of dispersion compensation and action as a line which becomes a part of an optical transmission line. That is, in order to cause the optical compensating optical fiber to function as an optical transmission line, the length of a dispersion compensating optical fiber is made one-third or more the length of an optical fiber having a positive dispersion value at a wavelength band of 1.55 xcexcm such as a single mode optical fiber; the dispersion compensating optical fiber is connected to an optical fiber having a positive dispersion value at a wavelength band of 1.55 xcexcm; the dispersion compensating optical fiber is caused to assume the roles of dispersion compensation and an optical transmission line, which transmits optical signals while compensating dispersion of the optical signals propagating through an optical fiber having a positive dispersion value at a wavelength band of 1.55 xcexcm; a line in which an optical fiber having a positive dispersion value at a wavelength band of 1.55 xcexcm and a dispersion compensating optical fiber are connected to each other is made into a new line of a wavelength division multiplex transmission line; and the present inventor propose the idea of a dispersion compensating optical fiber enabling construction of such a new line, and a new wavelength division multiplexed optical transmission line using such a dispersion compensating optical fiber.
The present invention was developed on the basis of the above ideas, and it is therefore a first object of the invention to provide a dispersion compensating optical fiber concurrently having the function of dispersion compensation and the function as an optical transmission line, and a second object of the invention to provide a wavelength division multiplexing transmission line which is composed by connecting an optical fiber having a positive dispersion value at a wavelength band of 1.55 xcexcm and the dispersion compensating optical fiber to each other and has a lower non-linearity, a lower loss characteristic than those in the prior arts, and excellent optical transmission properties.
In order to achieve the abovementioned objects, the invention is featured in the following constructions. That is, a first dispersion compensating optical fiber of the invention is a dispersion compensating optical fiber which compensates a dispersion characteristic of an optical fiber having a positive dispersion value at a wavelength band of 1.55 xcexcm, which is featured in that the dispersion value and dispersion slope are negative at a wavelength band of 1.55 xcexcm, the transmission loss is 0.3 dB/km or less, the polarization mode dispersion value is 0.15 ps/kmxc2xd or less, and the mode field diameter is 5.5 xcexcm or more.
A second dispersion compensating optical fiber of the invention is featured in that, in addition to the construction of the first dispersion compensating optical fiber, the dispersion value a at a wavelength band of 1.55 xcexcm is xe2x88x9250 ps/nm/km xe2x89xa6"sgr" less than xe2x88x9220 ps/nm/km.
A third dispersion compensating optical fiber of the invention is featured in that, in addition to the construction of the first dispersion compensating optical fiber, the dispersion value a at a wavelength band of 1.55 xcexcm is xe2x88x9220 ps/nm/km xe2x89xa6"sgr"xe2x89xa6xe2x88x9210 ps/nm/km.
Further, a fourth dispersion compensating optical fiber of the invention is featured in that, in addition to the construction of the second dispersion compensating optical fiber, the transmission loss at a wavelength band of 1.55 xcexcm is 0.25 dB/km or less.
Further, a fifth dispersion compensating optical fiber of the invention is featured in that, in addition to the construction of the third dispersion compensating optical fiber, the transmission loss at a wavelength band of 1.55 xcexcm is 0.25 dB/km or less.
A sixth dispersion compensating optical fiber of the invention is featured in that, in addition to any one of the constructions of the first through the fifth dispersion compensating optical fibers of the invention, the outer circumferential side of the center core is covered with a side core whose refractive index is lower than that of the center core, and the outer circumferential side of the side core is covered with cladding whose refractive index is higher than that of the side core but lower than that of the center core, wherein 1.0%xe2x89xa6xcex941xe2x89xa61.4% and xe2x88x920.45xe2x89xa6xcex942/xcex941 less than xe2x88x920.285 are created where the relative refractive index difference of the center core with respect to the cladding is xcex941 and the relative refractive index difference of the side core with respect to the cladding is xcex942, and 0.4xe2x89xa6Raxe2x89xa60.5 where a value (a/b) of the outer diameter a of the center core to the outer diameter b of the side core is defined as Ra.
Further, a seventh dispersion compensating optical fiber of the invention is featured in that, in addition to any one of the constructions of the first through the fifth dispersion compensating optical fibers of the invention, the mode field diameter is 6.2 xcexcm or more.
Further, an eighth dispersion compensating optical fiber is featured in that, in addition to the construction of the sixth dispersion compensating optical fiber, the mode field diameter is 6.2 xcexcm or more.
Still further, a first wavelength division multiplexing transmission line of the invention is a wavelength division multiplexing transmission line constructed so that an optical fiber having a positive dispersion value at a wavelength band of 1.55 xcexcm and any one of the first through the eighth dispersion compensating optical fibers are connected to each other, wherein a dispersion value at the wavelength band of 1.55 xcexcm is more than and including xe2x88x921 ps/nm/km but less than and including 1 ps/nm/km.
A second wavelength division multiplexing transmission line of the invention is a wavelength division multiplexing transmission line constructed so that an optical fiber having a positive dispersion value at a wavelength band of 1.55 xcexcm and any one of the first through the eighth dispersion compensating optical fibers are connected to each other, wherein a deviation of the dispersion values at the wavelength band of 1.55 xcexcm is 1 ps/nm/km or less.
A third wavelength division multiplexing transmission line of the invention is a wavelength division multiplexing transmission line constructed so that an optical fiber having a positive dispersion value at a wavelength band of 1.55 xcexcm and any one of the first through the eighth dispersion compensating optical fibers are connected to each other, wherein the optical fiber having a positive dispersion value is a single mode optical fiber having zero dispersion at a wavelength band of 1.31 xcexcm, and the length of the dispersion compensating optical fiber is approx. one-third or more of the length of the single mode optical fiber.
Further, a fourth wavelength division multiplexing transmission line of the invention is featured in that, in addition to the construction of the first wavelength division multiplexing transmission line, the loss at a portion of connecting an optical fiber having a positive dispersion value to a dispersion compensating optical fiber is 0.4 dB or less.
Still further, a fifth wavelength division multiplexing transmission line is featured in that, in addition to the construction of the second wavelength division multiplexing transmission line, the loss at a portion of connecting an optical fiber having a positive dispersion value to a dispersion compensating optical fiber is 0.4 dB or less.
Further, a sixth wavelength division multiplexing transmission line is featured in that, in addition to the construction of the third wavelength division multiplexing transmission line, the loss at a portion of connecting an optical fiber having a positive dispersion value to a dispersion compensating optical fiber is 0.4 dB or less.
Further, a seventh wavelength division multiplexing transmission line of the present invention is featured in that, in addition to the constructions of any one of the first through the sixth wavelength division multiplexing transmission lines, an intermediate mode field optical fiber having a mode field diameter which is between the mode field diameter of an optical fiber having a positive dispersion value and the mode field diameter of the dispersion compensating optical fiber is connected between the optical fiber having a positive dispersion value and the dispersion compensating optical fiber, and the length of the intermediate mode field optical fiber is made into 1/1000 or less of the length of an optical fiber having a positive dispersion value directly connected to the intermediate mode field optical fiber.
An eighth wavelength division multiplexing transmission line of the invention is featured in that, in addition to the constructions of any one of the first through the sixth wavelength division multiplexing transmission lines, an intermediate mode field optical fiber having a mode field diameter which is between the mode field diameter of an optical fiber having a positive dispersion value and the mode field diameter of the dispersion compensating optical fiber is connected between the optical fiber having a positive dispersion value and the dispersion compensating optical fiber, and the length of the intermediate mode field optical fiber is made into 1 m or more but 5 m or less.
Further, a ninth wavelength division multiplexing transmission line of the invention is featured in that, in addition to the construction of the seventh wavelength division multiplexing transmission line, the intermediate mode field optical fiber is a dispersion shifted optical fiber having zero dispersion at a wavelength band of 1.55 xcexcm.
Further, a tenth wavelength division multiplexing transmission line of the invention is featured in that, in addition to the eighth wavelength division multiplexing transmission line, the intermediate mode field optical fiber is a dispersion shifted optical fiber having zero dispersion at a wavelength band of 1.55 xcexcm.
In the invention, for example, a wavelength division multiplexing transmission line is formed by connecting a dispersion compensating optical fiber of the invention to a single mode optical fiber having zero dispersion at a wavelength band of 1.33 xcexcm by a length equivalent to approx. one-third (for example, approx. one-third times through one time) of the length of the single mode optical fiber. Where a wavelength division multiplexing transmission is carried out using optical signals of a wavelength band of 1.55 xcexcm using the wavelength division multiplexing transmission line, the respective wavelengths of a wavelength band 1.55 xcexcm gradually increase the positive dispersion in line with propagation in the single mode optical fiber.
Although wavelength division multiplexed optical signals of the respective wavelengths are caused to propagate, changing the single mode optical fiber to the wavelength compensating optical fiber, the dispersion compensating optical fiber of the invention has a negative dispersion slope at a wavelength band of 1.55 xcexcm, and the dispersion value "sgr" at the abovementioned wavelength band is a negative dispersion value in a range of xe2x88x9250 ps/nm/kmxe2x89xa6"sgr" less than xe2x88x9220 ps/nm/km or xe2x88x9220 ps/nm/kmxe2x89xa6"sgr"xe2x89xa6xe2x88x9210 ps/nm/km. The dispersion value of the single mode optical fiber at a wavelength band of 1.55 xcexcm is approx. 17 ps/nm/km. Therefore, by connecting a dispersion compensating optical fiber of the invention to the single mode optical fiber by a length exceeding approx. one-third of the length of the single mode optical fiber, in compliance with the dispersion value, the dispersion increased through propagation in the single mode optical fiber is compensated by the dispersion value a of the dispersion compensating optical fiber in a nullifying direction while propagating in the dispersion compensating optical fiber. And, optical signals can be received in a state where the dispersion of the respective wavelengths in multiplexed wavelength divisions is nullified and compensated at the terminal end of the dispersion compensating optical fiber.
Further, since a dispersion compensating optical fiber of the invention has a negative dispersion slope at a wavelength band of 1.55 xcexcm, a positive dispersion slope at a wavelength band of 1.55 xcexcm of a single mode optical fiber is nullified by the dispersion compensating optical fiber of the invention.
Still further, a dispersion compensating optical fiber of the invention is not a short moduled optical fiber as in the prior arts, to function not only to compensate the dispersion of a single mode optical fiber, but also as an optical fiber for optical transmissions. Therefore, the absolute value of a dispersion value at a wavelength band of 1.55 xcexcm as described above is made smaller than the absolute value of the dispersion value of the prior arts short moduled optical fiber, whereby the conditions for regulating the refractive index profile are slackened, and, in line therewith, since the mode field diameter can be increased, it is possible to form a dispersion compensating optical fiber provided with a non-linear property. Therefore, a dispersion compensating optical fiber of the invention can suppress distortions of the waveforms in wavelength division multiplexing transmission, and becomes suitable for laying cables.
Still further, since, in the dispersion compensating optical fiber of the invention, the transmission loss at a wavelength band of 1.55 xcexcm is 0.3 dB/km or less (preferably, 0.25 dB/km or less), the polarization mode dispersion value at the wavelength band of 1.55 xcexcm is 0.15 ps/kmxc2xd or less, and the mode field diameter is 5.5 xcexcm or less, the respective properties such as the transmission loss property, polarization mode dispersion property, etc., become excellent as a dispersion compensating optical fiber. Accordingly, when a wavelength division multiplexed light is caused to pass through the dispersion compensating optical fiber, the light can be transmitted without any hindrance due to loss and distortions (distortions due to chromatic dispersion and polarization mode dispersion) equivalent to those when the wavelength division multiplexed light is caused to pass through a dispersion shifted optical fiber having a zero dispersion wavelength at a wavelength band of 1.55 xcexcm, which is presently used. Therefore, a dispersion compensating optical fiber of the invention becomes suitable for laying cables in terms of this point.
Moreover, since a dispersion compensating optical fiber of the invention has a mode field diameter of 5.5 xcexcm or more (more preferably 6.2 xcexcm), it is possible to reduce the connection loss due to a difference between the mode field diameter of a single mode optical fiber and that of the dispersion compensating optical fiber. Accordingly, the transmission loss of a wavelength division multiplexing transmission line constructed by connecting a single mode optical fiber with a dispersion compensating optical fiber can be decreased, whereby a high bit rate and high rate wavelength division multiplexing transmission of high quality can be brought about.
Further, a dispersion compensating optical fiber of the invention is formed so that the outer circumferential side of the center core is covered with a side core whose refractive index is lower than that of the center core, the outer circumferential side of the side core is covered with cladding whose refractive index is higher than that of the side core but is lower than that of the center core, and the refractive index profile of the dispersion compensating optical fiber is, for example, a W-shaped profile. Therefore, it is possible to easily produce optical fibers of a refractive index structure, which are provided with the above established conditions of a dispersion compensating optical fiber. Furthermore, if the refractive index profile of a dispersion compensating optical fiber is established so that the range of xcex941 is 1.0%xe2x89xa6xcex941xe2x89xa61.4%, and the range of xcex942/xcex941 is xe2x88x920.45xe2x89xa6xcex942/xcex941xe2x89xa6xe2x88x920.285 where the relative refractive index difference of the center core with respect to the cladding is xcex941 and the relative refractive index difference of the side core with respect to the cladding is xcex942, and the range of Ra is 0.4xe2x89xa6Raxe2x89xa60.5 where a value (a/b) of the outer diameter a of the center core to the outer diameter b of the side core is defined as Ra, it is possible to inexpensively provide a dispersion compensating optical fiber which can be easily produced and has excellent features as described above, and a wavelength division multiplexing transmission line using the dispersion compensating optical fiber. And, both a low non-linear property and a low bending loss property can be securely achieved. Therefore, it is possible to sufficiently increase the optical transmission property of a wavelength division multiplexing transmission line constructed by connecting a dispersion compensating optical fiber of the invention and an optical fiber having a positive dispersion value at a wavelength band of 1.55 xcexcm to each other.
Still further, since a wavelength division multiplexing transmission line using a dispersion compensating optical fiber of the invention is formed by connecting a single mode optical fiber having zero dispersion at, for example, a wavelength band of 1.3 xcexcm, and the dispersion compensating optical fiber of the invention having a length exceeding approx. one-third of the single mode optical fiber to each other, the wavelength division multiplexing transmission line can greatly contribute to construction of an excellent wavelength division multiplexing transmission system in which the dispersion property at a wavelength of 1.55 xcexcm is flat, has a low non-linear property and a reduced bending loss, and by which distortions of transmitted wavelength division multiplexed light are remarkably reduced.
Further, by the connection loss at a connection portion between an optical fiber having a positive dispersion value at a wavelength of 1.55 xcexcm and a dispersion compensating optical fiber being made less than 0.4 dB, the loss of the entire optical transmission line can be made slight. For example, where the length of an optical fiber having a positive dispersion value at a wavelength band of 1.55 xcexcm is made into 20 through 30 km, the length of a dispersion compensating optical fiber is made into 20 through 10 km, the entire length of the optical transmission line is made into 40 km, and the transmission losses of the optical fiber having a positive dispersion value at a wavelength band of 1.55 xcexcm and the dispersion compensating optical fibers are, respectively, made into approx. 0.2 dB/km, the abovementioned connection loss becomes one-twentieth of the transmission loss of the entire length of the optical transmission line. Therefore, the connection loss is so small as can be disregarded, with respect to the loss level of the optical transmission line.
In other words, where the connection loss is 0.4 dB, it is equivalent to a loss equivalent to a length of 2 km of an optical fiber whose transmission loss is approx. 0.2 dB/km, the length of 2 km of an optical fiber is equivalent to only one-twentieth of the entire length of an optical transmission line when the entire length thereof is 40 km. Therefore, the connection loss is so small as can be disregarded, with respect to the loss level of the entirety of an optical transmission line.
Still further, according to a wavelength division multiplexing optical transmission line of the invention in which an intermediate mode field optical fiber having a mode field diameter which is between the mode field diameter of an optical fiber having a positive dispersion value at a wavelength band of 1.55 xcexcm and the mode field diameter of the dispersion compensating optical fiber is connected between the optical fiber having a positive dispersion value and the dispersion compensating optical fiber, a value obtained by adding a connection loss generated due to a difference between the mode field diameter of the intermediate mode field optical fiber and that of the dispersion compensating optical fiber to the connection loss generated by a difference between the mode field diameter of an optical fiber having a positive dispersion value at a wavelength band of 1.55 xcexcm and that of the intermediate mode field optical fiber becomes smaller than the connection loss generated by a difference between the mode field diameter of the optical fiber having a positive dispersion value at a wavelength band of 1.55 xcexcm and that of the dispersion compensating optical fiber.
Therefore, by causing the intermediate mode field optical fiber to intervene between the optical fiber having a positive dispersion value at a wavelength band of 1.55 xcexcm and the dispersion compensating optical fiber, it becomes possible to make small the loss of a wavelength division multiplexing transmission line. In particular, in those in which the length of the intermediate mode field optical fiber is made less than 1/1000 of the length of an optical fiber having a positive dispersion value at a wavelength band of 1.55 xcexcm, which is directly connected to the intermediate mode field optical fiber, influences of a dispersion slope which the intermediate mode field optical fiber has are not given to a wavelength division multiplexing transmission line formed by an optical fiber having a positive dispersion value at a wavelength band of 1.55 xcexcm and a dispersion compensating optical fiber of the invention. Therefore, it is possible to maintain a low loss property and a flat dispersion property at the wavelength band of 1.55 xcexcm and to construct an excellent wavelength division optical transmission system with excellent properties of the abovementioned dispersion compensating optical fiber.
Further, in those in which an intermediate mode field optical fiber is connected between an optical fiber having a positive dispersion value at a wavelength band of 1.55 xcexcm and a dispersion compensating optical fiber, and the length of the corresponding intermediate mode field optical fiber is more than approx. 1 m but less than approx. 5 m, an effect of suppressing the connection loss due to a difference between the mode field diameter of an optical fiber having a positive dispersion value at a wavelength band of 1.55 xcexcm and that of a dispersion compensating optical fiber can be securely brought about, and it becomes easier still to incorporate an intermediate mode field optical fiber in an optical transmission line as a connection portion. In particular, where the length of the intermediate mode field optical fiber is short from approx. 1 m or more through approx. 5 m or less, the intermediate mode field optical fiber can be formed in the form of a module, whereby the intermediate mode field optical fiber can be further easily incorporated.
Further, a dispersion shifted optical fiber having zero dispersion at the wavelength band of 1.55 xcexcm may be listed as an intermediate mode field optical fiber. Especially, where a single mode optical fiber having a zero dispersion wavelength at a wavelength band of 1.3 xcexcm is used as an optical fiber having a positive dispersion value at a wavelength band of 1.55 xcexcm, the effect of reducing the connection loss can be increased.